On design patterns: When should I use the singleton?
The glorified global variable - becomes a gloried global class. Some say breaking object-oriented design.
Give me scenarios, other than the good old logger where it makes sense to use the singleton.
On my quest for the truth I discovered that there are actually very few "acceptable" reasons to use a Singleton.
One reason that tends to come up over and over again on the internets is that of a "logging" class (which you mentioned). In this case, a Singleton can be used instead of a single instance of a class because a logging class usually needs to be used over and over again ad nauseam by every class in a project. If every class uses this logging class, dependency injection becomes cumbersome.
Logging is a specific example of an "acceptable" Singleton because it doesn't affect the execution of your code. Disable logging, code execution remains the same. Enable it, same same. Misko puts it in the following way in Root Cause of Singletons, "The information here flows one way: From your application into the logger. Even though loggers are global state, since no information flows from loggers into your application, loggers are acceptable."
I'm sure there are other valid reasons as well. Alex Miller, in "Patterns I Hate", talks of service locators and client side UI's also being possibly "acceptable" choices.
Read more at Singleton I love you, but you're bringing me down.
A Singleton candidate must satisfy three requirements:
If your proposed Singleton has only one or two of these requirements, a redesign is almost always the correct option.
For example, a printer spooler is unlikely to be called from more than one place (the Print menu), so you can use mutexes to solve the concurrent access problem.
A simple logger is the most obvious example of a possibly-valid Singleton, but this can change with more complex logging schemes.
读取只应在启动时读取并将其封装在Singleton中的配置文件。
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